The specific sorption of Np(V) on the corundum (α-Al2O3) surface in the presence of trivalent lanthanides Eu(III) and Gd(III): a batch sorption and XAS study

The sorption of pentavalent neptunium, Np(V), on corundum (α-Al2O3) was investigated in the absence and presence of trivalent europium or gadolinium as competing element under CO2-free conditions. The objective of this study was to investigate how a trivalent metal ion with a higher charge than that of the neptunyl ion would affect the sorption of Np(V) when allowed to adsorb on the mineral surface before the addition of Np(V). Batch sorption experiments conducted as a function of pH (pH-edges) and as a function of Np(V) concentration (isotherms) in the absence and presence of 1×10-5 M Eu(III) showed no sign of Eu being able to block Np sorption sites. Surface complexation modelling using the diffuse double layer model was employed to the batch data to obtain surface complexation constants for the formed Np(V) complexes on corundum. To account for potential changes occurring in the coordination environment of the neptunium ion in the presence of a trivalent lanthanide, X-ray absorption spectroscopic (XAS) studies of samples containing only Np(V) or Np(V) and Gd(III) were conducted. The XAS-measurements reveal the presence of a bidentate Np(V) edgesharing complex on the corundum surface in the absence of Gd(III). In the presence of Gd(III) our Np(V) EXAFS data show a contraction of the Np-Al distance together with the formation of an additional peak that is not resolved in the absence of the competing metal. These differences might point toward a change in the Np(V) surface configuration on corundum when Gd(III) is added to the sample before Np(V).